Robotic systems have been recently entered the medical arena for enhancing the surgeons' ability to precisely and minimally invasively position surgical tools. In particular, they have been used for remote manipulation (e.g. the daVinci® system supplied by Intuitive Surgical Inc., of Sunnyvale, Calif.), as semi active devices for brain biopsies applications (the NeuroMate™ system, supplied by Integrated Surgical Systems Inc., of Davis, Calif.) and as an active robot for hip and knee replacement (e.g. the ROBODOC® system, supplied by Integrated Surgical Systems Inc., of Davis, Calif.).
Failure of a positional control component could have serious repercussions in such hazardous tasks. In order to increase system reliability, prior art surgical robots have often been equipped with a double set of encoders or position sensors, these being the components that measure joint motions and provide the inputs for the control algorithms that determine the surgical tool position and orientation, i.e. the robot pose, and hence the motion path. The double set of sensors serve as a backup in case of an encoder failure. A discrepancy between the reading on the control encoder and its parallel back-up encoder immediately points to the failed sensor.
In serial type robots, where the links and joints are connected in series, each joint actuator affects the end-effector location serially and there is generally no internal position sensor that measures the end-effector location. Hence each encoder needs to be backed up by a second encoder on the same axis.
In a parallel type robot, on the other hand, and also in hybrid parallel-serial robots, it is possible to directly measure the end-effector location relative to the base and hence to locate a second set of back-up sensors not necessarily at the joints themselves but rather between the base and the output end-effector.
In the PCT application entitled “Precision Robot with Parallel Kinematics and a Redundant Sensor System” to M. Wapler, published as International Publication No. WO 01/19272, it is suggested that for a parallel robot with six degrees of freedom, it is possible to provide an acceptably safe backup for sensor failure using a minimum of three additional sensors disposed between the base and the moving platform.
However, since the cost of each position sensor and its associated control circuitry, is not insignificant, and even more importantly, since the space available in such miniature robots is at a premium, it would be desirable to devise a simpler method of providing back-up information for such robots, yet still providing an adequate safety margin.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.